The present invention relates to an active anti-vibration apparatus used for a semiconductor exposure apparatus for printing the circuit pattern of a reticle on a semiconductor wafer, a liquid crystal substrate manufacturing apparatus, or an electron microscope and, more particularly, to an active anti-vibration apparatus capable of suppressing external vibration transmitted to an anti-vibration table and positively canceling vibration generated by precision equipment placed on the anti-vibration table, and an exposure apparatus having the same.
In an electron microscope using an electron beam or a semiconductor exposure apparatus represented by a stepper, an X-Y stage is mounted on an anti-vibration apparatus. This anti-vibration apparatus has a function of attenuating vibration using a vibration absorption means such as an air spring, a coil spring, or an anti-vibration rubber member. Such a passive anti-vibration apparatus can damp vibration transmitted from the floor, though it cannot effectively damp vibration generated by the X-Y stage itself mounted on the apparatus. More specifically, a reaction force generated when the X-Y stage itself moves at a high speed swings the anti-vibration apparatus, and this vibration greatly impedes the positioning settling of the X-Y stage. Additionally, in the passive anti-vibration apparatus, the insulation (anti-vibration) performance for vibration propagating from the floor is a trade off with the suppression (vibration suppression) performance for vibration generated when the X-Y stage itself moves at a high speed. To solve these problems, active anti-vibration apparatuses are widely used A; in recent years. An active anti-vibration apparatus can eliminate a trade off between anti-vibration and vibration suppression within the range of an adjustable mechanism. Especially, performance which cannot be attained by a passive anti-vibration apparatus can be obtained by positively applying feedforward control.
Not only in a passive anti-vibration apparatus but also in an active anti-vibration apparatus, when an X-Y stage mounted on an anti-vibration table moves in step-and-repeat or step-and-scan operation, the barycenter changes due to movement of the stage, and the anti-vibration table tilts. This tilt is eliminated after the elapse of a sufficient time. However, since the step-and-repeat or step-and-scan operation is performed at a high speed, the anti-vibration table cannot restore its position in time, and eventually, the anti-vibration table tilts. Although this tilt is an inevitable physical phenomenon, it is disadvantageous for a semiconductor exposure apparatus. For example, when a functional unit (not shown) in the main body structure vibrates due to a tilt of the main body structure, predetermined performance cannot be obtained. As a measure, the natural frequency of the anti-vibration table is raised, i.e., the anti-vibration table is made rigid to suppress response to disturbance. In this case, however, vibration from the floor is readily transmitted to the upper portion of the anti-vibration table, resulting in degradation in anti-vibration characteristics. With this background, a demand for a technique of correcting a tilt of the main body structure without degrading the anti-vibration characteristics has arisen.
To help understand this situation, the above contents will be described with reference to the mechanical arrangement of an active anti-vibration apparatus having an X-Y stage mounted on an anti-vibration table. FIG. 7 shows the schematic mechanical arrangement of an active anti-vibration apparatus. Referring to FIG. 7, reference numeral 21 denotes an X-Y stage mounted on an anti-vibration table 22; and 23-1, 23-2, and 23-3, active supports supporting the anti-vibration table 22. Each active support 23-n (n=1, 2, 3) incorporates acceleration sensors AC, position sensors PO, pressure sensors PR, servo valves SV, and air spring actuators AS in number necessary for control in two axes, the vertical and horizontal directions. A suffix added to AC, PO, or the like represents direction (X,Y,Z) along the coordinate systems in FIG. 7 and the position of the active support 23-n (n=1, 2, 3). For example, Y2 indicates an element arranged along the Y axis and incorporated in the active support 23-2 on the front left side of the page.
A detailed description of the structure will be omitted. A phenomenon that takes place when the Y stage of the X-Y stage 21 moves in the Y-axis direction in FIG. 7 by a certain distance and stops will be described. For the active supports 23-n, Y-axis movement of the Y stage changes the barycenter of the entire anti-vibration table. The thrusts to be generated by the vertical actuators AC-Zn (n=1, 2, 3) in the active supports 23-n, which are necessary to maintain the horizontal posture of the anti-vibration table 22, are uniquely determined. When the Y stage has moved and stopped, and a sufficient time has elapsed, thrusts corresponding to the change in barycenter are generated by the active supports 23-n because of position control, and any tilt of the anti-vibration table 22 is removed. That is, the surface of the anti-vibration table 22 maintains its level. However, the situation changes when the Y stage continuously moves in the step-and-repeat or step-and-scan operation. When the Y stage continuously moves, the barycenter also continuously changes. Since the active supports 23-n cannot return to predetermined positions in time, the anti-vibration table 22 gradually tilts. When the X stage moves in the step-and-repeat or step-and-scan operation, rotation (tilt) about the Y axis occurs due to the same reason as described above. Such tilt of the anti-vibration table 22 may degrade the measuring accuracy of a measuring device (not shown) or the position settling o-of the stage itself to lower the productivity of a semiconductor exposure apparatus. Under the circumstance, a demand for a technique of correcting a tilt of the anti-vibration table due to a change in barycenter upon movement of the stage has arisen.
As a prior art for solving the above problems, Japanese Patent Laid-Open No. 9-134876 (anti-vibration apparatus and exposure apparatus) is known. According to this prior art, a tilt of the anti-vibration table due to a change in barycenter upon movement of the stage is predicted on the basis of an output from a stage position detection means (laser interferometer), and a command value for correcting this tilt is feedforward-input to the vibration control system of the anti-vibration apparatus. A voice coil motor (VCM) is used as an actuator, through which a steady-state current for correcting any tilt of the anti-vibration table due to movement of the stage flows. As can be easily understood by those skilled in the art, supply of a steady-state current has the following shortcomings.
(1) The VCM driving power supply becomes bulky. PA0 (2) The VCM and a power amplifier for driving the VCM generate heat. PA0 (3) A cooling unit for removing heat from the VCM and power amplifier must be prepared. PA0 (4) A humidity control apparatus for the entire semiconductor exposure apparatus becomes bulky.
Hence, it is preferable not to supply a DC current to a VCM for anti-vibration/vibration suppression of a large structure such as a semiconductor exposure apparatus. There are anti-vibration tables supporting a large structure without contact using an electromagnetic force. Such an anti-vibration table is disclosed in, e.g., Japanese Patent No. 2522736 (anti-vibration apparatus). However, this apparatus uses, as an actuator, an electromagnet as a magnetic bearing. This actuator normally steadily generates a force, and its use form cannot be put in the same category with that of the above-described VCM. A VCM is originally used by supplying a current to form a damping actuator for suppressing vibration of a mechanical structure. An air spring actuator capable of supporting a large mass by opening/closing a servo valve should be used for a work that requires a force. That is, a tilt of the anti-vibration table caused by moving load due to movement of the stage is preferably corrected by an air spring actuator. However, a technical problem still exists in realizing moving load correction. This is because an air spring actuator including a servo valve substantially has integral characteristics.
To help understand this further, an explanation will be given with reference to the drawings. FIG. 2 is a block diagram showing feedforward input to an astatic system. The astatic system represents integral characteristics 15 of an air spring actuator including a servo valve. Referring to FIG. 2, a correction signal corresponding to the stage position is input to a feedforward input terminal 14. When this signal is multiplied by an appropriate gain k.sub.p and feedforward-input, a mechanical system 16 of the anti-vibration table, i.e., a tilt of the anti-vibration table may seem to be correctable. However, since an air spring actuator including a servo valve substantially has integral characteristics, an actual driving force f obtained by feedforward-inputting the correction signal which linearly changes in accordance with the stage position is obtained by integrating the input correction signal. Apparently, a force for suppressing tilt of the anti-vibration table, which is proportional to the movement of the stage, cannot be generated. That is, with the arrangement for moving load correction as shown in FIG. 2, a tilt of the anti-vibration table cannot be corrected.
To generate a force corresponding to the stage moving position, the astatic system of the air spring actuator including a servo valve is changed to a static system. The static system represents characteristics 17 of, e.g., a system with time-lag of first order as shown in FIG. 3. With such characteristics, when a correction signal that linearly changes in accordance with the stage position is supplied to the feedforward input terminal 14, a driving force f corresponding to the stage position can be obtained. The time-lag of first order can be realized by pressure feedback to the servo valve substantially having the integral characteristics. The arrangement of pressure feedback is disclosed in Japanese Patent Application No. 9-68995 previously filed by the present applicant. However, this prior art aims at suppressing deformation of the main body structure. More specifically, it has a main object to prevent distortion of an exposure apparatus main body 13 by controlling the pressure of an air spring actuator. This prior art also discloses an arrangement for inputting so-called stage reaction force feedforward to an air spring actuator in the horizontal direction to suppress instantaneous swing due to acceleration/deceleration of the stage. The present invention provides not a control apparatus for suppressing such an instantaneous force but an apparatus arrangement for inputting feedforward input for suppressing a tilt of the anti-vibration table due to movement of the stage, or variation with a relatively low frequency, to an air spring actuator in the vertical direction. More specifically, the present invention provides an apparatus arrangement for correcting a tilt of the anti-vibration table effectively using pressure feedback described in Japanese Patent Application No. 9-68995 and on the basis of the stage moving position information.
In a semiconductor exposure apparatus, an X-Y stage that has large acceleration/deceleration is mounted on an active anti-vibration table. As the X-Y stage moves, the barycenter of the main body structure also moves. When this main body structure is supported by an active anti-vibration apparatus, the anti-vibration table is returned to a predetermined position by position control. However, when the moving amount of the X-Y stage is large, the moving time is short, and the movement is frequent, the anti-vibration table cannot return to the predetermined position in time and tilts as the X-Y stage moves at a high speed. A tilt of the anti-vibration table may vary the X-Y stage positioning characteristics in units of moving positions to lower the productivity.